Potentiometer contact springs

ABSTRACT

A method of manufacturing miniature potentiometer contact springs by winding a substantial length coil of a single layer of fine, heat-treatable precious metal alloy wire on a generally cylindrical form, masking spaced strips longitudinally of the wire coil, plating the wire coil between the masked strips, removing the strips and cutting the strip-plated wire band from the mandrel as a sheet, cutting the wire sheet to produce contact strip blanks of various forms severable into individual contacts. A multi-layer masking system is used to photographically define the spaced strips. An adherent support about the wire coil permits the coil to be removed before masking and processed in sheet form. Contact springs using wire of a reduced lateral diameter are produced by flattening the wire prior to winding on the form.

United States Patent [191 Dieterich [1 1 3,733,573 1 May 15, 1973 [54]POTENTIOMETER CONTACT SPRINGS [76] Inventor: Frank L. Dieterich, 3979East Boulevard, Los Angeles, Calif. 90010 [22] Filed: May 24, 1971 [21]Appl. No.: 146,360

Related U.S. Application Data [63] Continuation-in-part of Ser. No.728,683, May 13,

1968, Pat. No. 3,579,822.

[52] U.S. Cl ..338/202, 29/l93.5, 29/630 E [51] Int. Cl. ..H0lc 1/12[58] Field of Search ..338/202, 171, 174,

338/162, 183, 176; 29/l93.5, 193, 630 E; 310/248, 251; 200/l68 BHPrimary Examiner-Lewis H. Myers Assistant Examiner-D. ToneAttorney-Fulwider, Patton, Rieber, Lee & Utecht [5 7] ABSTRACT A methodof manufacturing miniature potentiometer contact springs by winding asubstantial length coil of a single layer of fine, heat-treatableprecious metal alloy wire on a generally cylindrical form, maskingspaced strips longitudinally of the wire coil, plating the wire coilbetween the masked strips, removing the strips and cutting thestrip-plated wire band from the mandrel as a sheet, cutting the wiresheet to produce contact strip blanks of various forms severable intoindividual contacts. A multi-layer masking system is used tophotographically define the spaced strips. An adherent support about thewire coil permits the coil to be removed before masking and processed insheet form.

Contact springs using wire of a reduced lateral diameter are produced byflattening the wire prior to winding on the form.

17 Claims, 14 Drawing Figures I. III...

PATENTEDNAHSIQYS 73 SHEET 2 BF 3 MANDL 1 POTENTIOMETER CONTACT SPRINGSCROSS-REFERENCE TO RELATED APPLICATIONS This is a Continuation-In-Partof US. Ser. No. 728,683, filed May 13, 1968 now US. Pat. No. 3,579,822,and entitled METHOD AND BLANK FOR MAKING POTENTIOMETER CONTACT SPRINGS.

BACKGROUND OF THE INVENTION 1. This invention is directed to multiplecontact point contact springs for miniature potentiometers in whichcontact fingers engage a resistance surface under pressure and areslidably movable therealong to effect electrical contact therewith atthe points of contact finger engagement and to methods for making suchcontact springs.

2. Potentiometer contact springs have long been made from thin sheets orstrips of heat-treatable metal alloy, with the contact slotted or slitto provide a plurality of fingers to ensure full contact and engagementwith the resistance surface. In miniature potentiometers, the contactbecomes very small and it is difficult to slit it into the small fingerwidth and also to provide the slot widths without increasing the overallwidth of the contact. Punching of the slits in a die has becomeimpractical because of the narrowness of the slits and fingers and thevery short life of dies which will produce such narrow slits and contactfingers. Narrow slits can be produced with electron beam cutting butthis requires an extremely costly machine which must be operated in avacuum and involves other production difficulties.

It has, therefore, been proposed to produce multifinger contact springsof small size by using small round or rectangular wires which are placedparallel to each other and joined together at one end, with their freeends forming the multiple contact fingers. Such multiple wire contactsare shown in the US. Pat. to Raymer No. 2,760,036 and Louis et al. No.3,328,707.

The method employed by Raymer is described inhis specification incolumns 8 and 9 under the central heading C. Manufacture of wiper brush.Raymer winds his wire upon a carbon rod and plates the entire wire coilwith copper. He then removes the plated band from the rod and coats theportion of the plating which is to remain intact with an acid-resistingmaterial and then removes the copper from the remainder of the wire bynitric acid. Thereafter the contactingend is formed and the contactmounted. The Raymer process is subject to several disadvantagesincluding the limiting of his contact wires to platinum because of theacid treatment, the fact that he can plate on only one side of the wirecoil, and must plate the entire circumference of the coil. His method isconsiderably more complex, expensive and time-consuming than that of thepresent invention.

In the Louis et al. patent, the contact wire springs are handledindividually and are individually mounted in clips or clamps in whichthey are welded and soldered. In view of the small size of theindividual wiper wires, such manipulation is extremely difficult andtimeconsuming and is not suited to high production output.

In addition to the above, potentiometer contact springs in generalsuffer from noise problems. In this regard, it is known that increasingthe number-of wire contact fingers decreases the amount of noise andimproves the contact spring efficiency. Typically, this is accomplishedby using a smaller diameter finger, so that the number of contact pointsis increased without increasing the lateral width of the contact spring.However, lesser diameter fingers present their own inherent problems.

First, it has been shown that the pressure per finger should beapproximately in the range of 3 to 7 grams, preferably about 5 grams.Greater pressure could tend to excessively abrade the contact point,while lesser pressure could tend to introduce noise. As the fingerdiameter is decreased, the finger stiffness correspondingly decreasesuntil the finger can no longer provide the desired contact pressure.While the length of the wire finger could be reduced to provide greaterstiffness, this length is typically itself only about 0.05 inches andreducing it would decrease the finger flexibility and present verycritical manufacturing tolerances and increased costs. Moreover, itbecomes more difficult and costly to draw wire smaller than a givendiameter of about 0.002 inches, so that use of such extremely fine wire,even if feasible, becomes unduly expensive.

SUMMARY OF THE INVENTION In its broad aspect the present inventionprovides methods of economically forming high-quality spring contacts inlarge quantities without the drawbacks of the prior art. To this end thecontacts are formed by winding a coil layer of wire on a form, maskingthe coil to provide circumferentially spaced, longitudinal coil stripswhich receive conductive metal deposits. The coil is cut longitudinallyand removed from the form as a sheet which is processed as contact stripblanks of end-to-end or side-by-side connected spring contacts fromwhich individual contacts are severable.

In another aspect of the invention, the masking may be by photographictechniques to provide highly accurate delineation of the spaced strips.A photosensitive resist masking system is applied to the coil, exposedthrough a negative of the desired strip pattern, and developed to fromthe strips.

In one modification, the process of the invention includes applying anadherent support to the coil and then removing the coil from the form asa sheet of wire fingers held together by the support. Subsequentprocessing, including the masking and metal strip deposition, can beaccomplished on this flat sheet with the support being removed at apoint subsequent to the metal deposition.

More specifically, in preferred embodiments according to the presentinvention, the contact spring is made from a heat-treatable metal alloywire which is plated adjacent its attachment end, leaving free multiplewire fingers which are to make contact with the potentiometer resistancesurface. The wire is wound on a mandrel and is masked so that theplating metal, such as silver, copper, or nickel, will be deposited inspaced strips running longitudinally of the wire coil on the outside ofthe coil.

The mask may be a photosensitive resist wrapped around the coil as amulti-layer system having a resist layer and a protective layertherefor. The strip pattern negative is wrapped and fastened, as byadhesive, around the resist system or, alternatively, is produceddirectly adherent to the protective layer as a silkscreened image or adeveloped photosensitive emulsion layer, the emulsion having beenexposed by light to which the resist is insensitive.

The coil is removed from the mandrel in the form of a sheet which isthen cut transversely of the wires at the center portions of both theplated and unplated strips, resulting in long contact strip blanks ofshort wires bonded at one end and free at the other, or is cut parallelto the wires to form blanks of severable end-to-end connected contacts.The free ends of the wires may then (or after individual contacts arecut off) be formed to radiused points of contact and heat-treated inconventional manner at a temperature and for a time to develop hardness,wear resistance and spring characteristics. The sheet or contact blanksmay form articles of commerce with the customer slicing the strip intothe individual contacts or performing any other required processing. Inany case, the individual contacts are formed at their plated portions,if desired, and assembled to a support. The mask may remain on andprovide protection for the delicate wires, at least until the wires areformed or heat-treated.

To process the coil in sheet form, including the steps of masking andmetal deposition, the adherent support is applied around the coilexterior in the form of, for example, an adherent lacquer or piece ofadhesive tape, the lacquer or tape tightly securing the wire fingers.The wire surfaces formerly interior of the coil are exposed in sheetform for subsequent processing.

The metal strip deposition can be performed on both sides of the coil bywrapping the coil about a slotted tube, the deposition to the insidecoil surface being through the tube slots, the tube thereby functioningas a mask.

In another aspect of the invention, the wire fingers of 5 a contactspring have a reduced diameter in the direction of the lateral contactwidth. In this embodiment, the wire may be flattened, for example,between rollers, prior to being wound on the form. This operationproduces a barrel-shaped wire with relatively flat opposing sides alongthe wire length, the barrel height connecting the flat sides beingreduced from the original wire diameter, while the barrel diameter is increased. With this structure the number of fingers for a given widthcontact spring is greater, providing more contact points, while the widebarrel diameter, in the direction of the wire finger flexure, stiffensthe fingers to maintain proper contact pressure on the resistancesurface.

The above invention lends itself to high production automationprocedures and is superior in economy and in the performance of thecontact spring product. It does not require handling of parts smallerthan the finished contact and enables the accurate definition andproduction of contacts of any desired size in a standard contact strip.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a progressive, perspectiveview showing the steps in the process of forming potentiometer contactsprings according to the present invention;

FIG. 2 is a view partly in section and partly in side elevation showingthe contact mounted in a potentiometer in engagement with a resistancesurface;

FIG. 3 is a partial longitudinal sectional view on the line 3-3 of FIG.1 showing plating deposited on the outside of the wire coil whilewrapped on a mandrel;

FIG. 4 is a partial cross-sectional view on the line 4-4 in FIG. 1showing wire wound on a mandrel and covered by a multi-layer maskingsystem;

FIG. 5 is a progressive, perspective view showing application of anadherent negative image to the multilayer masking system;

FIG. 6 is a progressive, perspective view showing another technique forapplying an adherent image to the masking system;

FIG. 7 is a progressive, perspective view showing a modification of theprocess of FIG. 1 for processing the wire coil in sheet form;

FIG. 8 is a perspective view showing a slotted tube used in plating onboth sides of the wire coil;

FIG. 9 is a perspective view of the wire band obtained by the platingmethod of FIG. 8;

FIG. 10 is a partial longitudinal sectional view on the line 10-10 ofFIG. 8;

FIG. 1 1 is a perspective view of a modified tube mandrel, wire coil andmasking prior to plating;

FIG. 12 is a partial longitudinal sectional view on the line 8-5 of FIG.1 showing several turns of the wire coil wrapped on a mandrel;

FIG. 13 is a partial longitudinal sectional view similar to FIG. 12showing several turns of barrel-shaped wire wrapped on the mandrel; and

FIG. 14 is a cross-sectional view through a barrelshaped wire formed byflattening round wire and shows in dotted outline the original size ofthe round wire.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in the drawings, forpurposes of illustration, and in particular FIGS. 1, 2, and 3 thereof,the present invention is embodied in a process of forming contactsprings for miniature potentiometers using a wire coil 11 wound on atube mandrel 12 as shown in FIG. 1a, the coil being processed in thesteps depicted in the figures to form contact springs having a pluralityof wire fingers.

The coil 11 of bare wire is wound spirally in immediate turn by turncontact upon the tube 12, the tube being of conductive material such asstainless steel. The wire may be of any desired size and composition, anexample being a heat-treatable alloy of about 32 percent palladium, 27percent silver, 17 percent copper, 14 percent platinum, 9 percent gold,remainder zinc, this alloy commercially available under the name PA-LINEY from the J. M. Ney Company of Bloomfield, Conn. While the size ofthe wire will depend upon the performance and physical requirements ofthe finished contact, a typical example in a miniaturized potentiometerwould be wire of a diameter in the range of 0.002 0.005 inches; however,wire of a diameter greater or less than this range may be processedaccording to the present invention, if desired.

A first process according to the invention is depicted in step sequenceof FIGS. 1a, 1b, and 1f through II. There the wire wound tube 12 isdipped in a nonconducting rubber or plastic base platers maskingsolution 13, shown in FIG. 1b, to completely cover the tube and wire andthe resulting mask 14 is thoroughly dried and, thereafter, strips 15removed longitudinally of the coil to leave circumferentially spacedbare strips 16 on the wire coil, as shown at FIG. If, on whichconductive metal plating will be deposited.

The resulting strip, coil and mandrel are placed in an electroplatingbath 17 surrounded by a cylindrical anode 18 of silver which isconnected to the positive terminal of a direct current voltage source.The negative terminal of the direct current source is connected at 19through the mask directly to the tube 12 so that contact is made throughthe tube to the wire coil 11. Silver is then deposited on the wire coilin strips 20 to any desired thickness, for example, 0.002 0.005 inch, asshown in FIG. 3. Alternatively, the deposition may be by electrolessplating or other suitable metal deposition techniques. In addition,while silver has been utilized for the strips 20, copper or nickel arealso satisfactory materials.

The plated wire coil and the tube mandrel are then removed from the bathl7 and the mask removed (FIG. 1h) by mechanical or chemical peeling,leaving nonplated coil strips 21. However, the mask need not be removedat this point, and in many cases it is desirable to leave the mask on toprotect the delicate wire coil fingers during subsequent processing,handling, and shipping. In any event, the wire coil is then cutlongitudinally, for example in the middle of a strip 21, and is removedfrom the mandrel and straightened to form a sheet 22 having spacedstrips of silver plating 20 with strips 21 of unbonded wiretherebetween. The band or sheet 22 is then sheared into strips by cuts23, preferably through the middle of the unbonded strips 21, and cuts 24through the middle of the plated strips 20 as shown in FIG. lj'. Thisresults in a plurality of individual contact strip blanks 25 having freeunbonded wires 26 at one edge and bonding plating 27 at the other edge.The cuts 23 and 24 need not necessarily be made through the middle ofrespective strips 21 and 20; however, this is less difficult than, forexample, cutting a line at the intersection of a strip 21 and theadjacent strip 20.

At this point alternate procedures may be followed in which either thestrip blank 25 is formed and heattreated while intact to produce anarticle of commerce where the customer will slice the blank at lines 28into individual contact widths or the strip blanks are immediately cutalong lines 28 to form individual contacts 29 and 30 of any desiredwidth and the forming of the free ends of the wires 27 and theirheat-treatment performed on the individual contacts. For either case,the forming of the wire fingers into radius points of contact isindicated in the dies 31 and 32, which operation should be done beforeheat-treating the contact fingers in conventional manner at atemperature and for a time to develop hardness, wear resistance andspring characteristics. Any forming of the plated portion of the contactmay be done after heat-treating, if desired, because the silver preventsany spring back from changing the set of the form. The finished contactmay be spot welded at 33 to a support 34 as shown in FIG. 2 where thecontact wires 26 have radius ends 35 contacting a potentiometerresistance surface 36 on a ceramic substrate 37. The spring fingers 26engage the surface 36 under pressure and are slidable thereacross by thesupport 34 in conventional manner.

Instead of forming a forminga strip blank 25 by cuts 23 and 24perpendicular to the wire fingers as in FIG. lj', the sheet 22 may beout along lines 38 parallel to the wire fingers, as in FIG. lj, to formstrip blanks 39 comprising end-to-end connected spring contacts 29 whichmay be severed therefrom by a cut 40 in step 1K and thereafter formedand heat-treated as previously described. The cuts 38 may be spaced anydesired amount with the strip blank 39 having a lateral width greaterthan a single contact 29, if desired, in which case the cut wouldproduce a strip blank similar to blank 25 requiring a cut 28 to sever anindividual contact 29 therefrom.

Because of the minute sizes of the contact springs, it is important thatthe bare strips 16 along the coil 11 and the metal strips 20 depositedthereon be equally, circumferentially-spaced and in absolutelongitudinal parallelism. To this end, a modified process according tothe invention is shown in steps b-e in FIG. 1 in which photographicmasking is utilized to define the longitudinal strips.

After winding the coil 11 on the tube 12 in step l-a, a photosensitiveresist is applied around the exposed circumference of the coil at l-b.While the resist may be applied wet by spraying or dipping, in apreferred form it is applied by wrapping a dry, multi-layer maskingresist system 41 around the coil. Thereafter, a negative 42 is wrappedaround the resist system at l-c. After exposing the resist through thenegative at l-d, all layers, including negative 42, radially outward ofthe resist are removed at l-e, and the resist is chemically developed atl-f to remove the strips 15.

One satisfactory resist system, sold under the Trademark RISTON,comprises a photosensitive resist layer 43 sandwiched between a 1 mil,transparent polyethylene cover sheet and a 1 mil, transparent polyesterprotective layer 44, the layer 44 being specifically of the polyestersold under the Trademark MYLAR. The cover sheet is removed and discardedand the duallayer of resist 43 and protective MYLAR 44 is wrapped aroundthe coil 11 as shown in FIG. 4, the tube 12 having been previouslyheated to about 220250F. At this temperature, the resist becomes tackyand is pressed into the grooves between the wires and is adherent to thewires.

The photographic negative 42, with the longitudinal strip pattern imageformed thereon, is then rolled onto the protective layer 44 after theapplication of a thin, transparent adhesive coating to the layer 44which serves to secure the negative in close, adherent contact to theprotective layer. The tube 12 is then rotated before a collimated lightsource 45 of 3,500A. to expose the resist 43 through the negative.Thereafter, the protective layer is peeled away as shown in step becarrying the negative with it, and the resist is developed in a bath oftrichloroethane which dissolves the unexposed resist strips 15 leavingthe coil bare in strips 16 for previously described plating step l-g.Prior to plating, a suitable non-conductive mask 14 (FIG. 3) is appliedto the tube areas not covered by the coil 11 to prevent metal depositionthereon.

In an alternative process of the invention, the necessity of wrappingthe negative 42 around the protective layer 44 is eliminated bydepositing the negative image directly on the protective layer prior towrapping the resist system 41 around the coil 11. As showndiagramatically in FIG. 5, the resist system 41 is laid flat and thenegative image is silk-screened directly on the protective layer 44using conventional screening inks and techniques. The negative isadherent to and, thus, is in the closest possible contact with theprotective layer, and as a result, the image definition is controlledentirely by the sharpness of the silk-screening. After applying theresist system with the adherent negative image to the coil, the processof steps l-d through l-l is carried out.

The resist system can be made entirely photographic by applying aphotosenstive emulsion 45 to the protective layer 44 as shown in FIG. 6.The emulsion is exposed with collimated light 46 through a negative 47of the desired longitudinal strip pattern and then developed, so thatthe protective layer has the pattern image adherent thereto. Afterwrapping the resist system and adherent negative image around the coil11, the coil is processed according to steps l-d through l-e. The resist43 should be insensitive to light of the wavelength used to expose theemulsion 45.

According to another aspect of the invention, shown in FIG. 7, the coil11 may be removed from the tube 11 as a sheet prior to masking steps l-bor l-b' and processed thereafter in sheet form. After winding the coilon the tube, an adherent support 48 is applied to the coilcircumference. The support may be an adherent lacquer sprayed on thecoil and allowed to dry. In another form, the support may be a piece ofadhesive, plastic tape wrapped around the coil. The tape should have awidth w spanning all turns of the coil 11. The support 48 and coil arethen slit longitudinally, and the coil is flattened in sheet form. Thesupport holds the wire fingers together, and the finger surfacesoriginally interior of the coil flush against the tube are thus exposedin planar fashion. This sheet 49 of wire fingers is then processed as inFIGS. 1 through 6, beginning with masking step l-b or l-b', the onlydifference being that the sheet is flat, not cylindrical, and thuseasier to handle. To ensure electrical connection to all wire fingers inthe plating operation l-g in the absence of tube 12, a suitable shortingcontact or clip should be applied spanning and contacting all fingers.The adherent support 48 can be stripped from the sheet at any pointafter the plating operation, since the metal strips then provideadequate support to the fingers. The support 48 should withstand thechemicals or other materials used in the masking and plating operations,so that the support afforded the wire fingers is not impaired.

A modified process which plates the wire coil and contacts on both sidesis illustrated in FIGS. 8-10. Here a cylindrical mandrel 51 is providedwith a plurality of longitudinally extending, circumferentially spacedslots 52 therethrough and the mandrel 51, which may again be ofstainless steel, is dipped in the masking solution 13 and dried prior tothe winding of a wire coil 53 thereon. The width of the slots 52 aftermasking is the desired width of the plated bands 54 on the wire coil(FIG. 9). The wire coil 53 is then wound around the cylindrical mandrel51 and the masking solution is painted on the wire coil to cover onlythe area of the bridges of the slotted cylinder as at 55.

The masked coil and mandrel are then placed in a plating bath, as at 17,with an outer cylindrical anode, the same as 18 in the description ofthe FIG. 1 illustrated process, and also with an axial silver rod anode56, the anodes 18 and 56 being connected together to the positive sideof a direct current voltage source and the wire coil 53 itself beingconnected directly to the negative side of the direct current source.Plating then occurs on the outside of the wire coil between the maskingstrips 55 and on the inside of the masked coil through the slots 52 toprovide plating, as shown in FIGS. 9 and 10, at 54a on the outside andat 54b on the inside of the wire coil. The platings 54a and 54b are eachof the order of 0.001 0.002 inches of silver.

After plating, the mask on the wire coil is peeled off and the coil cutthrough the middle of an unplated wire strip and removed from thecylindrical mandrel, as shown in FIG. 9, whereupon the band is developedinto a sheet as indicated at 22 in FIG. 1 and the remaining operationscarried out as described in the process illustrated in FIG. 1. The maskis desired beneath the wire coil 53 with the slotted mandrel 51 toprovide a resilient surface on which the wire is wound to preventcreeping of the plating up the wire from the slot area.

A further modification of the process is illustrated in FIG. 11 whereina wire coil 61 is wound on a non conducting tube mandrel 62, of glass orthe like, and is masked by means of strips 63 of platers masking tapewhich are adhered to the outside surface of the coil in longitudinallyextending strips circumferentially spaced to provide bare wire strips 64therebetween to which the plating is applied in a plating bath asillustrated in FIG. 1. In this arrangement, the overall masking step isavoided as is the stripping of the mask from the coil where plating isdesired. The coil itself will be connected to the negative terminal ofthe dc. voltage source and anode 18 to the positive side as in theplating bath 17. After plating, the masking tape strips 63 are readilypeeled off and the band cut from the mandrel and developed into thesheet 22 and processed as previously described.

The previous embodiments utilized conventional wire of roundcross-section. This is depicted in FIG. 12 where several turns of wirecoil 11 are shown in a crosssectional view through the coil 11 and thetube 12 on which the coil is wound.

Referring to FIGS. 13-14, another embodiment of the invention is shownin which contact springs are produced with wire having a reduceddiameter portion. Prior to winding the wire of coil 11 on tube 12, thewire is flattened, resulting in a wire cross-section 69 of barrel-shapehaving opposing flat surfaces between which the barrel height H" ismeasured and opposing arcuate surfaces, at 180 to the flats, spacedapart by the barrel diameter D. This may be accomplished by passing thewire between two rollers (not shown) which flatten the wire on opposingsides. As shown in FIG. 14, the flattening of originally round wire 11of original diameter X produces relatively flat, opposing wire surfaces70 and 71 spaced apart by the height H." In addition, the wire has topand bottom arcuate circumferential surfaces 72 and 73, respectively,spaced apart by the diameter D and connecting the surfaces 70 and 71.

The barrel-shaped wire is wound on the form 12 as shown in FIGS. 13 withthe flat surfaces 70 and 71 of each wire turn in lateral, side-by-siderelation and with the bottom arcuate surface 73 in wrapped contact withthe 'tube. Since the height H of each wire turn is less than theoriginal diameter of the wire 11, it is evident that for a givenlongitudinal distance along the tube 12, more wire turns are producedusing the barrel-shaped wire than the original round wire. Forexample,FIG. 12 shows eight turns of round wire 11, while FIG. 13 shows 12 turnsof barrel-shaped wire 69 in the same space along the tube 12. As aresult, a contact spring made from the coil of FIG. 13 has more contactpoints then would a coil of FIG. 12 of the same lateral width. Inaddition, it is significant that the flattening process does not destroythe arcuate circumference at the contact spring contact point. Thus,either surface 72 or 73 presents a desirable small area of contact whenformed into an electrical contact point for an individual contactspring.

The winding may be accomplished by positioning the two rollersvertically, close to the tube 12, so that the wire is oriented properlywhen received by the tube. In addition, a finger (not shown) may bearagainst each wire turn when received by the tube to press it securelyagainst the preceding turn.

Contact springs are made with barrel-shaped wire using the previouslydescribed processes of FIGS. l-ll, the only difference in the processingbeing the shape of the wire.

In addition to providing more wire fingers for a given width contactspring, the barrel-shaped wire of height H is inherently stiffer than around wire with a diam eter equal to the height H. This is evident sincethe barrel wire diameter D is greater than the round wire diameter, andthe barrel wire thus is wider in the direction of fiexure of the wirefinger to maintain proper contact pressure on a resistance surface.

In one example, 0.003 inch diameter round wire of the PALIN EY alloypreviously mentioned was rolled to produce a barrel-shaped wire having aheight H of 0.002 inches and a diameter D of 0.0041 inches. Theflattening also elongated the wire about 3 percent. The relative valuesof D and H will depend, of course, on the type of wire used. A springcontact with this 0.002 inch height barrel-shaped wire would have thesame number of fingers as an identical width contact of 0.002 inchdiameter round wire; however, the barrel wire is stiffer because of itsincreased diameter D of 0.0041 inches.

The barrel-shaped wire can be produced having a height less than 0.002inches, if desired. In this form, it can be used as extremely minutecontact spring fingers of height H 0.001 inch or less at a costconsiderably less than round wire of the same size while simultaneouslyproviding a stiffer, better contacting finger than the round wire.

In the above-described embodiments of the invention, the tube 12 mayhave a diameter of about 8 inches and the wire coil 11 may be wound for12 inches longitudinally along the tube. This would produce a sheet 22of about 12 inches by 24 inches or about 288 square inches. A typicalindividual contact spring measures 0.1 by 0.1 inches or 0.01 squareinches. Thus, the above sheet would contain about 28,800 individualcontact springs. The diameter of tube 12 and the length that coil 11 iswound can be less than or even greater than the above values to producesheets 22 of various desired sizes.

It will be apparent from the foregoing that while particular forms ofthe invention have been illusand described, various modifications can bemade without departing from the spirit and scope of the invention.Accordingly, it is not intended that the invention be limited except asby the appended claims.

I claim:

I. An intermediate product comprising:

a contact strip blank for spring contacts comprising a plurality ofside-by-side, relatively fine wire fingers, said wire fingers beingjoined by plural, spaced, continuous bonded strips of conductive metal,said strip blank including a plurality of endto-end connected springcontacts severable therefrom by cuts perpendicular to said wire fingers.

2. An intermediate product as defined in claim 1, wherein said wirefingers are of a precious metal alloy substantially 0.0020.005 inches indiameter.

3. An intermediate product as defined in claim 1, wherein said strips ofconductive metal are formed from the group including silver, copper, ornickel platmg.

4. An intermediate product as defined in claim 1, wherein said blankfurther includes a plurality of sideby-side connected ones of saidcontacts severable therefrom by cuts parallel to said wire fingers.

5. An intermediate product as defined in claim 1, wherein the portionsof said wire fingers between said strips of conductive metal are coveredby a nonconductive, protective mask.

6. An intermediate product as defined in claim 1, wherein thecross-section of said wire fingers is barrelshaped, the barrel heightbeing less than the barrel diameter, with the barrel height aligned toposition the relatively flat wire finger surfaces in side-by-siderelatron.

7. A potentiometer contact spring, comprising:

a plurality of relatively fine wire fingers positioned side-by-side in alateral direction, said fingers having a small-area, electricalcontacting surface near one end thereof, said fingers having a reduceddiameter in said lateral direction between opposing lateral sides ofeach finger.

8. A potentiometer contact spring as defined in claim 7, wherein saidopposing sides are relatively fiat.

9. A potentiometer contact spring as defined in claim 7, wherein thecross-section of said fingers is barrelshaped, said opposing sides areconnected by top and bottom arcuate, circumferential finger surfaces,one such circumferential surface forming said small-area electricalcontacting surface.

10. A potentiometer contact spring as defined in claim 9, wherein theheight of said barrel is said reduced diameter and the diameter of saidbarrel is the distance between said top and bottom surfaces, the barrelheight being less than the barrel diameter.

11. A potentiometer contact spring as defined in claim 10, wherein saidwire fingers are formed by flattening originally round cross-sectionwire, said barrel height being less than and said barrel diameter beinggreater than the original diameter of said round wire.

12. A potentiometer contact spring as defined in claim 10, wherein saidwire fingers are shaped in the axial direction of said fingers to formsaid small-area electrical contacting surface.

13. A potentiometer contact spring as defined in claim 12, wherein theshaping in the axial direction provides a radiused contact area.

14. A potentiometer contact spring as defined in claim 10, and furtherincluding:

lateral support means for joining said fingers at the end thereofopposite said one end to provide electrical interconnectiontherebetween, said fingers extending freely in independent fiexurerelation from said lateral support means.

15. A potentiometer contact spring is defined in claim 14, wherein saidsupport means is a metal layer selected from the group of silver,copper, or nickel.

16. A potentiometer contact spring as defined in claim 10, wherein saidbarrel height is less than about 0.002 inches.

17. A potentiometer contact spring as defined in claim 16, wherein saidbarrel height is less than about 0.001 inches.

1. An intermediate product comprising: a contact strip blank for springcontacts comprising a plurality of side-by-side, relatively fine wirefingers, said wire fingers being joined by plural, spaced, continuousbonded strips of conductive metal, said strip blank including aplurality of end-to-end connected spring contacts severable therefrom bycuts perpendicular to said wire fingers.
 2. An intermediate product asdefined in claim 1, wherein said wire fingers are of a precious metalalloy substantially 0.002-0.005 inches in diameter.
 3. An intermediateproduct as defined in claim 1, wherein said strips of conductive metalare formed from the group including silver, copper, or nickel plating.4. An intermediate product as defined in claim 1, wherein said blankfurther includes a plurality of side-by-side connected ones of saidcontacts severable therefrom by cuts parallel to said wire fingers. 5.An intermediate product as defined in claim 1, wherein the portions ofsaid wire fingers between said strips of conductive metal are covered bya non-conductive, protective mask.
 6. An intermediate product as definedin claim 1, wherein the cross-section of said wire fingers isbarrel-shaped, the barrel height being less than the barrel diameter,with the barrel height aligned to position the relatively flat wirefinger surfaces in side-by-side relation.
 7. A potentiometer contactspring, comprising: a plurality of relatively fine wire fingerspositioned side-by-side in a lateral direction, said fingers having asmall-area, electrical contacting surface near one end thereof, saidfingers having a reduced diameter in said lateral direction betweenopposing lateral sides of each finger.
 8. A potentiometer contact springas defined in claim 7, wherein said opposing sides are relatively flat.9. A potentiometer contact spring as defined in claim 7, wherein thecross-section of said fingers is barrel-shaped, said opposing sides areconnected by top and bottom arcuate, circumferential finger surfaces,one such circumferential surface forming said small-area electricalcontacting surface.
 10. A potentiometer contact spring as defined inclaim 9, wherein the height of said barrel is said reduced diameter andthe diameter of said barrel is the distance between said top and bottomsurfaces, the barrel height being less than the barrel diameter.
 11. Apotentiometer contact spring as defined in claim 10, wherein said wirefingers are formed by flattening originally round cross-section wire,said barrel height being less than and said barrel diameter beinggreater than the original diameter of said round wire.
 12. Apotentiometer contact spring as defined in claim 10, wherein said wirefingers are shaped in the axial direction of said fingers to form saidsmall-area electrical contacting surface.
 13. A potentiometer contactspring as defined in claim 12, wherein the shaping in the axialdirection provides a radiused contact area.
 14. A potentiometer contactspring as defined in claim 10, and further including: lateral supportmeans for joining said fingers at the end thereof opposite said one endto provide electrical interconnection therebetween, said fingersextending freely in independent flexure relation from said lateralsupport means.
 15. A potentiometer contact spring is defined in claim14, wherein said support means is a metal layeR selected from the groupof silver, copper, or nickel.
 16. A potentiometer contact spring asdefined in claim 10, wherein said barrel height is less than about 0.002inches.
 17. A potentiometer contact spring as defined in claim 16,wherein said barrel height is less than about 0.001 inches.